Speed of Reflected Light on a Rotating Plane Mirror

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A horizontal beam of light reflects off a rotating plane mirror at 30 revolutions per minute, with the nearest point on the screen located 20 meters away. To find the speed of the light spot on the screen, the angular velocity of the mirror is calculated as 30*2*Pi/60 radians per second. The velocity of the endpoint of a perpendicular rod at 20 meters is then determined, and the speed of the light spot is twice this value due to the geometry of the reflection. It is emphasized that drawing a diagram is crucial for understanding the problem and visualizing the distances involved. Properly analyzing the angles and distances will lead to the correct calculation of the light spot's speed across the screen.
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A horizontal beam of light is reflected from a plane mirror that revolves about a vertical axis at a rate of 30 rev/min. The reflected beam sweeps across a screen that, at the point nearest the mirror, is 20 m away. With what speed does the spot of light move across the screen at the point nearest the mirror?

Is it c? (3*10^8)
I know I am not understanding. Can someone explain this one to me? I am not even sure where to start.
 
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The angular velocity of the mirror is 30*2*Pi/60 radians per second. Multiply this by 20 meters to get the velocity (in meters per second) of the endpoint of a rod which is 20 meters long and perpendicular to the mirror. The speed of the spot of light will be twice that, that is, if I understood the problem correctly. Note that when the mirror (along with the imaginary attached rod) turns by 45 degrees, the reflected ray turns by 90 degrees, i.e. 2*45 degrees.
 
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Always start by drawing a diagram!
Make sure the quantity you want (distance along screen) is on the diagram!
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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